The purpose of this matabolic and postnatal study is to determine the interaction between two common environmental factors (physical activity and type of dietary carbohydrates) and placental growth rate, morphometric indices of placental functional capacity, size at birth and post-natal growth and developmemt in the human. Its basic strategy is to examine the impact of alterations in dietary carbohydrates and physical activity on multiple objective indices of growth and function. The significance of this study is that it will determine if feto-placental growth can be modified by a woman's diet and activity. If this is correct then it will be theoretically possible to alter these factors to treat disorders of placental and fetal growth.